Re: PCI domains

[Simon Burge <simonb%NetBSD.org@localhost>]

"Christoph Egger" wrote:

> > In message: <20090710024528.EC3695985C%thoreau.thistledown.com.au@localhost>
> >             Simon Burge <simonb%NetBSD.org@localhost> writes:
> > : Christoph Egger wrote:
> > : 
> > : > Michael Lorenz wrote:
> > : >
> > : > > +/*
> > : > > + * NetBSD's userland has a /dev/pci* entry for each bus but
> > userland has no way
> > : > > + * to tell if a bus is a subordinate of another one or if it's on a
> > different
> > : > > + * host bridge.
> > : > 
> > : > I have a patch which introduces support for PCI domains. It allows the
> > : > userland to distinguish between them by checking if the pci bus
> > belongs
> > : > to the same PCI domain.
> > : 
> > : What exactly is a "PCI domain"?  A quick google seems to suggest that
> > : this is a Linux concept as opposed to a PCI concept.  In a previous
> > : life we used NetBSD on a number of different machines of various
> > : architectures that had multiple PCI host bridges, although admittedly we
> > : didn't need to know the topology of the PCI bus layout.
> > 
> > PCI domains, as implemented by Linux and FreeBSD are separate host
> > bridges.  Each bus complex behind the host bridge has independent
> > numbering.  Each of these bus complexes are in a different domain.
> > 
> > Not all systems with multiple host bridges have separate numbering
> > domains, but many do.
> 
> PCI domains are part of the PCI host bridge specification.
> It is a 16bit-wide number.

Can you please provide a reference for this?  There's nothing mentioned
in the PCI Local Bus Specification version 2.3.  I can find plenty of
references to "clock domains" and "address space domains", but when I
exclude Linux references I can't seem to find anything useful.

Everything I see so far says that a "PCI domain" is an OS abstraction
and not part of a PCI specification.  I'm also not sure exactly what
problem you're trying to solve - why exactly does it matter if two PCI
busses share a common "domain" or not?

Cheers,
Simon.